The present invention relates to a method and circuitry for channel estimation in a CDMA cellular communication system.
The invention is particularly concerned with a cellular communication system in which signals are transmitted with variable data transmission rates wherein data is transmitted as a plurality of data symbols over a sequence of time slots. As is known, in a CDMA system data is encoded for transmission by modulating data symbols to be transmitted by a unique spreading code for each channel. Within each cell of a cellular communication system, spreading codes are preferably orthogonal to allow for a plurality of different mobile stations to communicate with a base station on selectively coded channels.
When a signal is transmitted between a base station and a mobile station (either on the uplink or the downlink), the signal receiving unit needs to establish from the signal which it has received some information about the communication path along which the signal has travelled. This is referred to herein as xe2x80x9cchannel estimationxe2x80x9d and is carried out in a channel estimation unit which generates a channel impulse response. Various techniques are known for channel estimation. The channel impulse response is required in order to properly decode and demodulate incoming data.
In an earlier proposed CDMA system explained in the Technical Report of IEICE (1996-02) entitled xe2x80x9cVariable Rate Data Transmission on Single Code Channels in DS-CDMAxe2x80x9d by Okomura et al, data is transmitted at variable transmission rates. Channel estimation is carried out from pilot symbols spaced periodically in the data stream.
At least two difficulties are associated with this proposal. One is that only the energy from the pilot symbols is available for channel estimation, and the signal to noise ratio of pilot symbols tends to be poor. Thus, filtering is required to get more reliable estimates. In particular in the case of mobile stations moving at high speeds, it may be difficult to apply a suitable filtering technique due to short channel coherence time.
Furthermore, pilot symbols are required only in coherent systems. In a non-coherent system, in which pilot symbols are not present there remains the difficulty of providing reliable channel estimation for variable data transmission rates.
According to an aspect of the present invention there is provided a method for channel estimation in a cellular communication system with variable data transmission rates wherein data is transmitted as a plurality of data symbols over a sequence of time slots, each time slot having at least a proportion containing data symbols, said proportion being dependent on the data transmission rate and wherein an estimate of the transmission rate is determined and is used in channel estimation so that the channel estimation is based on received data symbols.
The present invention can be used in a CDMA system with discontinuous pilot and variable data rates, because by estimating the transmission rate the channel estimation unit is aware of how much of the frame is filled with data symbols or whether a frame is empty. Thus, decision feedback base coherent channel estimation is possible using all the transmitted data symbols and not just transmitted pilot symbols.
The present invention can be utilised in a situation where data and rate information are encoded on a common code-defined channel for transmission, or where the rate information is transmitted on a parallel control channel. In either case, the estimate of transmission rate can be determined from that rate information.
The invention can also be used in a situation where no rate information is transmitted. In that case, the transmission rate can be estimated as one of a number, preferably small, of known possible transmission rates. The correct one can be determined by error checking procedures after decoding of data.
However, it is preferred if the estimate of transmission rate is determined further upstream in the signal processing sequence for received signals and thus in the preferred embodiment of the invention the estimate of transmission rate is based on the data structure in each time slot prior to decoding said rate information. For that purpose, an additional rate estimation unit is provided distinct from the rate detection unit which decodes the rate information.
The rate information could be derived from the spreading code structure on the data channel.
The estimate of transmission rate which is determined based on the data structure in each time slot can usefully be used in other parts of the received signal processing sequence. For example, it can be used to assist in rate selection for Viterbi decoding of the incoming data symbols, or to assist in decision of an alternative transmission rate for decoding if an error check fails.
The invention is applicable in a situation where pilot symbols are provided for coherent transmission, but also in a non-coherent communication system where no pilot symbols are transmitted.
According to another aspect of the present invention there is provided signal receiving circuitry for use in a cellular communication system with variable data transmission rates wherein data is transmitted as a plurality of data symbols over a sequence of time slots, each time slot having at least a proportion containing data symbols, said proportion being dependent on the data transmission rate, the signal receiving circuitry comprising:
a rate estimation unit for receiving incoming data symbols and determining therefrom an estimate of the transmission rate; and
a channel estimation unit which utilises the estimated transmission rate of the received data symbols in channel estimation.
The signal receiving circuitry can also include a rate detection unit which decodes rate information transmitted with the data symbols.
The signal receiving circuitry can be provided in a mobile station or base station.
In the rate estimation unit, detection of an estimated transmission rate can be done based on the data structure in each time slot as for low data rates part of the time slot is empty and the rate estimation unit can detect if transmission is switched off part of the slot. As an example, a 10 ms frame can consist of several slots which generally have the same structure, so the rate estimation unit can measure the breakpoint (switch off point) from several slots and average over all the slots. This information can be compared with the rate decoded from the rate information by the rate detection unit.
The following described embodiment of the present invention provides several advantages.
The energy of received data symbols in addition to that of pilot symbols is available for channel estimation.
The rate estimation unit is able to detect some properties of the data structure and thus knows when transmission in each frame should end. Thus, verification of the encoded rate information is allowed.
As the rate estimation unit provides additional information concerning the transmission rate, corrective actions can be taken on the basis of this. If the allowed number of transmission rates is relatively small, the rate estimation unit can determine the most probable actual transmission rate and further signal detection can proceed based on that information.
For a better understanding of the invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings.